The present invention relates to a hydraulic circuit, in particular for the traction of a hybrid motor vehicle, a method for controlling this hydraulic circuit, and a hybrid motor vehicle comprising such a hydraulic circuit.
A known type of transmission for a hydraulic hybrid motor vehicle, presented in particular in document FR-A1-2973302, comprises a planetary gear train comprising three elements connected to an internal combustion engine, a hydraulic pump and a differential powering the driving wheels. The transmission receives a hydraulic machine which can be operated as a motor or a pump, and which can be connected to the differential by several gear ratios.
The hydraulic circuit comprises a low-pressure circuit and a high-pressure circuit, each of the circuits comprising a pressure accumulator for storing energy, with the low-pressure accumulator maintaining a minimum pressure threshold to avoid cavitation of the hydraulic machines. The stored pressure fluids are then returned to the high pressure circuit to apply a driving torque to the wheels. Alternatively, the low-pressure circuit may include a booster pump which maintains a minimum level of pressure.
Various operating modes are thus obtained, including traction of the vehicle only by the hydraulic machine, the internal combustion engine being stopped, and traction by the internal combustion engine which delivers a torque to the differential and to the pump providing hydraulic power. In the latter mode, a complementary torque given by the hydraulic machine can be added.
A “short ratio” mode is also obtained with the pump stopped, the internal combustion engine delivering high torque to the drive wheels by the planetary gear train forming a speed reducer, and a “long ratio” mode is obtained with the internal combustion engine delivering a weaker torque to the driving wheels by the planetary gear train which is blocked. In addition, a “braking” mode is obtained where the hydraulic machine, working as a pump, delivers a braking torque from the vehicle, by recharging the high-pressure accumulator.
In addition, hydraulic power machines generate heat during the operation produced by the internal losses, causing a heating of the fluid. A heat exchanger is generally arranged along the main flow of fluid supplying the power to the various hydraulic machines.
This arrangement requires a heat exchanger of dimensions sufficiently large to limit the pressure drop at a high flow rate, which results in a large size and mass, whereas on vehicles it is desired to reduce these parameters in order to reduce energy consumption. Moreover, the cost of this large filter is also high.
In addition, there may be a problem of internal contamination of the circuit, in particular of the hydraulic machines, coming from materials introduced into the circuit during manufacturing of the machine, or from particles generated by the wear of the internal components during operation of the machine. This contamination leads to faster aging of the devices and can cause failures.
In order to eliminate impurities from the fluid, a filter is also generally provided in the main flow of power of the hydraulic circuit, which necessarily comprises a single direction of passage of the fluid.
However, in the case of a circuit comprising hydraulic machines working in rotation in both directions, such as for the transmission of the hybrid vehicle presented by the aforementioned prior art document, the single direction of passage in the filter may require components that rectify this direction, including an arrangement of several non-return valves, which adds bulk, mass and costs.
Furthermore, the filter, generally mounted along the main flow of the hydraulic circuit, must be of a significant size in order to avoid too great a pressure drop on this flow, which can be high.